Polyesters, especially poly(ethylene terephthalate) (PET), are versatile polymers that enjoy wide applicability as fibers, films, and three-dimensional structures. A particularly important application for PET is for containers, especially for food and beverages. This application has seen enormous growth over the last 20 years, and continues to enjoy increasing popularity. Despite this growth, PET has some fundamental limitations that restrict its application in these markets. One such limitation is related to its permeability to gases such as oxygen and carbon dioxide. A second limitation is related to the tendency for pressurized PET containers to fail catastrophically when exposed to certain chemicals (a phenomenon known as stress-crack failure). A third limitation is related to the tendency for PET to have a high coefficient of friction when in contact with itself.
A number of technologies have been developed to overcome these limitations. For example, in order to improve the barrier properties of PET, polyester co-polymers and blends have been developed, such as PET containing 2,6-naphthtalenedicarboxylic acid and PET/MXD6 blends. Other developments include the use of organic and inorganic barrier coatings (such as epoxy-amines and SiOx coatings) and multilayer structures containing barrier polymers (such as EVOH and MXD6). To improve the stress-crack resistance of PET, higher molecular weight PET has been commercialized. To improve the sliding ability of PET in contact with itself, low aspect ratio inorganic additives such as silica, talc, and zeolites have been employed. However, each of these technologies has drawbacks. Thus, all of the aforementioned barrier technologies add substantially to the cost of PET packaging. Higher molecular weight PET is more difficult to process and is more expensive to produce. Additives such as silica, talc, and zeolites, while decreasing the sliding friction of PET, also increase the haze of the polymer.
A technology that could address each of these limitations of PET involves incorporation of high-aspect ratio nanomaterials into PET to form PET nanocomposites. Nanocomposites are polymeric materials that contain a particulate additive which has at least one dimension substantially less than a micron. When the additive also possesses a high aspect ratio (aspect ratio is defined as the ratio between the average of the lateral dimensions and the particle thickness; the lateral dimensions being the length and width of the particle), the resultant nanocomposites can exhibit improved barrier properties because the high aspect ratio of the additive increases the tortuosity of the path that gas molecules must travel in permeating the polymer. PET nanocomposites may also possess improved resistance to stress-crack failure because high-aspect ratio additives can provide a mechanism to hinder crack propagation. PET nanocomposites may also possess a reduced coefficient of friction through roughening of the PET surface, or by providing a surface with a higher hardness and/or lower coefficient of friction than PET itself.
In addition to polyesters, a number of other polymers are used in applications where permeation of gases, water, or organic molecules is detrimental. For example, polyolefins are widely used to make pipes for natural gas transport, for gas tanks in automobiles, and for food packaging applications. Polydienes are used widely as rubber for structures such as pneumatic tires. Polyvinyls such as polyvinyl chloride, polystryrene, and acrylonitrile-butadiene-styrene (ABS) are frequently used in applications where enhanced barrier performance would be desirable. Polyamides, which are used as barrier layers in some of these applications, would also benefit from the enhanced barrier performance arising from the incorporation of high aspect ratio materials. In all of these materials, use of high aspect ratio nanomaterials is limited or non-existent, because of the difficulties associated with incorporating exfoliated phyllosilicates into these polymers and maintaining the phyllosilicates in an exfoliated state.
Essentially all of the high-aspect ratio materials previously developed for use in polymers are based on phyllosilicates such as montmorillonite, a naturally occurring layered aluminosilicate clay that possesses charge-balancing monovalent and divalent ions, as well as traces of transition metal ions such as iron. Because of the high charge density on phyllosilicates, the individual layers are strongly attracted toward each other. To obtain high aspect-ratio nanomaterials, these layers must be separated, or exfoliated. In order to achieve exfoliation in nonaqueous environments, the metal ions are exchanged with hydrophobic quartemary ammonium salts to produce organically-modified phyllosilicates. These organically modified phyllosilicates can then be exfoliated in relatively polar polymers such as nylon 6, nylon 6/6, and MXD6. The lateral dimensions of the high-aspect ratio phyllosilicates are on the order of 250 nanometers.
In spite of the potential for phyllosilicates to be used to enhance the barrier properties of PET, little progress has been made in achieving PET/phyllosilicates polymer compositions. This lack of success is due to the chemical nature of PET; unlike nylons, there are a number of undesirable side reactions that can occur during the polymerization or processing of PET that are catalyzed by various metal ions and/or amine-containing compounds. For example, incorporation of relatively low levels of monovalent or divalent metal ions into PET can result in the rapid nucleation of the PET, rendering processing difficult or impossible. Transition elements such as iron can contribute to generation of acetaldehyde and color. Quaternary ammonium salts decompose at the temperatures required to melt-process PET, resulting in amines which rapidly cause formation of color and diethylene glycol in the PET, as well as loss of molecular weight. Finally, degradation of the quaternary ammonium salts can cause the exfoliated phyllosilicates to reaggregate, with a resultant loss of the high aspect ratios required to achieve the desired properties in PET compositions.
A further limitation of the phyllosilicates is that at an aspect ratio of 250, loadings of 2–10 weight percent in the polymer are necessary to achieve significant barrier improvement factors. Thus, in order to increase the barrier performance of MXD6 nylon by a factor of 4 requires 3.5 weight percent of an exfoliated phyllosilicates. The need for these high loadings, and the cost associated with modifying and incorporating the phyllosilicates into polymers places significant constraints on the price of the nanomaterials. In fact, it is for this reason that most of the nanocomposite research has focused on the modification and use of naturally occurring, abundant montmorillonite clays.
It therefore would be advantageous to develop layered nanomaterials which possess aspect ratios substantially greater than that available in the phyllosilicates, which are chemically benign to polymers such as polyesters, and which are comparatively easy to incorporate and exfoliate into polymers. It would be a further advantage if these nanomaterials could be readily synthesized from high-purity raw materials. It would be even a greater advantage if the chemical structure, functionality, and physical dimensions of the nanomaterials could be readily controlled. One class of materials which has some members that meet these criteria are layered metal phosphonates. Layered metal phosphonates are a subset of all metal phosphonates. Depending on the reactants, stoichiometries, and synthesis conditions, metal phosphonates can also form one-dimensional chains, one-dimensional nanotubes, three-dimensional microporous frameworks, and non-porous three-dimensional frameworks.
Although layered metal phosphonates have been known for a number of years, there have been few attempts to incorporate them into polymers. Thus U.S. Pat. No. 4,232,146 discloses the preparation of layered tetravalent metal phosphonates, but does not disclose exfoliation of the metal phosphonates in a polymer matrix. French patent application 81 05797 discloses polyester compositions with improved crystallization rates that comprise a) a thermoplastic polyester, b) a nucleating agent chosen from metal salts of organophosphonic, organophosphinic, and organophosphonous acids, and c) a plasticizing agent. However, there is no teaching of the use of layered metal phosphonates or exfoliation of the metal phosphonate in the polyester. U.S. Pat. No. 4,759,971 discloses the use of layered tetravalent metal phosphonates as adhesion promoters in polymer matrices but does not disclose exfoliation of the tetravalent metal phosphonate. JP 48074550 describes the use of metal salts of arylphosphonates as nucleating agents for polyesters. Once again, there is no teaching of the use of layered metal phosphonates or exfoliation of the metal phosphonates.
A need remains, therefore, for improved polymer compositions that include metal phosphonates and for related processes and articles.